Methimazole for hyperthyroid - my cat has had a bad reaction!

jen2006August 7, 2010

I am really worried about my 13 year old cat. She has been diagnosed with hyperthyroid, and high blood pressure. The vet put her on methimazole. We started out with a lower dose, and all was well, she gained some weight and seemed to get some of her pep back. But, after a few weeks, she became one of the very small percentage of cats that will self mutilate on this medication. She began scratching constantly at her face and ears and head. Her eyes were all puffy, her ears full of scratches, blood everywhere...

So, the vet stopped the meds, gave her treatment for the irritation and inflammation, and when she was back to normal, we decided to try the meds again, only at very small doses, to be very very slowly increased. But, after a few weeks, even on a very low dose, she started scratching at her head and face and ears again.

I just don't think this med is going to work for her, and I am really worried, the only other option is I think surgery, but she is so thin, and the vet won't even do a dental on her till she gains some weight. She was never a big cat, even at her full weight she has always been trim and light. So, once she started losing weight due to thyroid issues, it didn't take long for her to get down to 7 lbs. I am so worried about her heart and other organs, I hear they can suffer under the effects of the disease she has.

I was wondering if anyone has had this problem with methimazole, and what they have done about it...or if anyone has any tips on how I can get some weight on the cat, or does the hyperthyroid make that impossible? I will be taking her back to the vet next week, but was hoping for some ideas and other experiences that maybe I could discuss with the vet.


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The only "cure" for hyperT is radioactive iodine (I-131), which cures the disease in 98-99% of cats. It is definitely the hyperT treatment of choice for cats with healthy kidneys and would certainly be the best option for any cat who can't tolerate thyroid medication. You can read about 1-131 at the link below.

Surgery is the most risky and perhaps least effective treatment for hyperT, since it may not remove all of the diseased tissue, causing the disease to remain or return after surgery.

As far as methimazole is concerned, exactly what "very low dose" caused your cat to have the bad reaction? It's possible that you may be able to cut the dose back even more and see how that works for your cat. Other options are to try the transdermal form of the drug (a gel that gets rubbed into the inner ear flap) or to try carbimazole (which converts into methimazole only AFTER it leaves the stomach). I don't know if either of those options would reduce or eliminate your cat's negative reaction to methimazole, but they might be worth trying. If these other med options don't work for her, then I-131 is definitely something to look into.

I doubt if you'll be able to put any weight on your girl until you successfully manage or cure her hyperT.


Here is a link that might be useful: radioactive iodine treatment for hyperT in cats

    Bookmark   August 7, 2010 at 10:38AM
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I opted for the radioactive iodine treatment for my cat a few years ago.

Emotionally, it's horrible because your cat is isolated from you and everyone else for a couple of weeks, but, in the long run, I think it's the best option.

When your cat gets home from the treatment, she'll be back to her old self in no time.

I'd look into it. I chose it because of all the side effects I'd read about with the medications.

Feel free to email me if you want to know about my (my cat's) experience with the treatment.

    Bookmark   August 7, 2010 at 1:21PM
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Time of isolation varies depending on the state regulations. Here in NC, you can get your cat back in 3 days, with some precautions to take regarding handling of slightly radioactive urine and stool. The I-131 treatment is just a single subcutaneous injection, not painful. Definitely look into it.

    Bookmark   August 7, 2010 at 6:07PM
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thanks for all of your replies. I believe my vet did mention the long term isolation time with the rad. iodine treatment, so it must be a few weeks in my state as well.

The dose for the methimazole started out at 2.5 mg, to be increased to 5 mg eventually, but I never got past the 2.5 dose the first time, after a few weeks is when the reactions started. We stopped the med, gave her system some time to clear out, then I started her again at a little less than a quarter tab ONCE a day...which would be I guess around 1 mg, give or take. The idea was to start her out at much lower dose and very very slowly increase vet said she had another patient who slowly acclimated her cat to the med this way and that it worked out fine, her cat's body adjusted well to the slow and gradual increases and she was able to eventually give her a full dose on a regular basis. I was hoping this would work with Liza, but after about 10 days, she began having the same skin reactions, though not as severe, on the roughly 1 mg dose. I suppose I could try again with half a mg.

I really do hate to have to be separated from her for so long with the rad. iodine treatment, but if she can't take the meds in any form (going to try the gel and the carbimazole if possible) it may be the only option, or her health will just deteriorate. Though she has high blood pressure, the vet said that all her systems seem to be pretty good for her age, so maybe we can do that. I have some thinking to do, and also some new ideas to discuss with the vet.

Thanks Laurie for the link, I am going to read that now. And thanks to everyone else as well, I appreciate your time to respond!

    Bookmark   August 7, 2010 at 9:51PM
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Which state are you in? I'll be happy to do a Google search and see what I can find out about isolation times for I-131 treatment of cats in your state. As difficult as it would be to be separated from your girl for I-131 treatment, the long term benefits for her and for you would be well worth it.

Methimazole and carbimazole are very serious drugs with very serious potential side effects both over the short and long terms. Even if your girl can acclimate to the drug over the next few months, these drugs have the potential to cause permanent liver and kidney damage over the long term. You need to keep that in mind when weighing your treatment options.

Just so you understand, there is no "full dose" of methimazole per se. The proper dose for your cat is whatever dose maintains her T4 hormone at the proper level. And since hyperT is a progressive disease, it's common for the dosage to need to be increased over time as the disease worsens. The safest strategy for determining the proper med dose is to start very low, maintain the low dose for 3 weeks, then retest and see what effect that dose has had on the T4 level. If the T4 level is not yet optimal, the dose is adjusted up or down slightly, as necessary, with another retest in 3 more weeks. That schedule of med adjustment followed by a retest in 3 weeks is maintained until the cat achieves euthyroid (normal T4 level), after which the cat is maintained on whatever dose achieved euthyroid. Retests are then performed every 3-6 mos to make sure the dose is still maintaining euthyroid. Many hyperT cats maintain euthyroid on very low doses, some as low as .625 mg daily. The goal is not to work your girl up to 5 mg a day. The goal is to achieve euthyroid at as low a dose as can do so.


    Bookmark   August 8, 2010 at 12:18AM
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Laurie, I am in New Jersey, southern half of the state. Burlington or Camden or Ocean County are convenient. Philadelphia is the closest large major city.

Thanks for taking the time to share this information with me. Hopefully it will also help others in the future if they have the same issue. My vet, I suppose because of the cat's weight, started me at 2.5 mg, but the bad reactions occurred before I got her back in to test her levels (to see if the dose was therapeutic). I was thinking of going down to one quarter of that dose-which would be .625 as you mentioned above- as a last resort with this med. However, what you say about the long term effects of this medication leaves room for more thinking about it. She is an older cat, and I don't want to do any damage to her organs.

So yes, if you could find out what the quarantine time is in NJ, I would appreciate that, or if you could tell me where to search for such info, that would be fine too.

You have been very helpful, and I am grateful. Do you work in the animal field?

    Bookmark   August 11, 2010 at 8:03PM
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I ran a Google search for I-131 treatment clinics in NJ, and it appears that you have quite a few of them. Most of them note a post-treatment hospitalization quarantine of only 4 days. One of them notes 4-7 days. So you've got a nice short quarantine period there in NJ.

The only "animal field" work I've done is as a riding instructor at my own small stable for a number of years. I also majored in Equestrian Studies in college several decades ago. Other than that, my animal education and experience has been provided by my rather extensive 4-legged family over the last 50+ yrs. There's not much they haven't taught me about.


    Bookmark   August 12, 2010 at 2:15AM
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I am going through the same thing right now with my 12 year old cat. She currently has a cone on her neck to prevent her from itching her eyes. They think it's from the Methimazole, but I stopped giving it to her a week ago. Her eyes are puffy and itchy and don't seem to be getting much better. Could you tell me how long it took for your cat to stop these symptoms after you discontinued the Methimazole? She is supposed to have the radioactive iodine treatment in less than two weeks, but if she doesn't get better soon she won't be able to have it yet. By the way, after doing so much research, I definitely recommend that treatment. It looks like the life expectancy of the cat is quite longer with the treatment than it is with the medication. I am so worried about her being isolated during treatment, but I think it will be best in the long run.
Will you please email me as well as post because I am not going to be checking this site as it was just an internet search. Thanks!

    Bookmark   August 17, 2010 at 10:29PM
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Laurie, that is GREAT!! If it is only a few days, or even a week, then I think that both I, and the kitty, can deal. I have her on .625 mg now, but I have alread noticed some fresh scratches on her ears...

Ashley, I feel for you!! So hard to see this. My cat on a higher dose had red puffy eyes, a scratch from the corner of her eye down to the tip of her nose, scratches all over her neck and ears, plus she is pure white so all of the usually pale pink skin you can see on white cats, such as their ears, would turn beet red. After her worst reaction, the vet gave her a shot of hydrocortisone and meds for me to give her at home and told me to keep her off all other meds, such as even low doses of the methimazole for about two weeks, so I believe it can take some time for the allergic reaction to subside and for them to get back to normal.

Aren't we the lucky ones to have one of those cats that fall into the 1% that have a bad reaction to this med!

Your cat should be ok soon, did you vet give her anything to ease the reaction? If not and she still seems to be suffering, I would ask. Please let me know how it goes with the iodine treatment.

And Laurie, thanks again. You have been a great help.

    Bookmark   August 18, 2010 at 6:54PM
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Jen & Ashley
I know this is somewhat an older post, but I have a few questions for you.
Peachy, my 14 year-old cat has been on Methimazole (1.5 mg a day-liquid) for 6 weeks. She will have her first post-med blood work lab on Monday. Remembering your experience with this drug, I was keeping a close eye on her. Last week, there was a scratch over the eye that healed. Today, I noticed a small bald area just at the lower corner of her eye. Do you remember if that's how it started for your cats? Not bad at this point, but I don't want it to get any worse.
Hope you both found an acceptable solution for your cats (besides the e-collar). Iodine treatment?

Hope you are doing well,

    Bookmark   September 23, 2010 at 10:53AM
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Hello, my cat Gabby was recently put on Methimazole for her Thyroid. All seemed to be going well but within the last few days I have noticed scratches above her eyes and in her ears. She also seems to be biting at herself like she itches all over. The vet did have her on 1 tablet per day but when she was not gaining enough weight upped it to 1 1/2 pills per day. When I noticed her puffy eyes and lackluster attitude I backed it down to 1 pill per day on my own. This morning I found her just sitting in the corner by the water bowl. She didn't sleep with us all night. I do have a call into my vet. In Wisconsin how long is the period of time she needs to be quarantined if she gets the Iodine injection. Thanks to anyone who can help. Ifeel so bad for Gabby that it makes me want to cry. PLEASE HELP!!

    Bookmark   May 6, 2013 at 8:28AM
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Scratching at the face is a known side effect of methimazole in some cats. If one of my hyperT cats had a similar reaction (which, thankfully, none of them have), I would stop methimazole immediately and discuss alternate treatments with my vet. Radioactive iodine is the only cure for hyperT, so it is the treatment of choice. Below is a link to the I-131 informational brochure of the UWM vet clinic.


Here is a link that might be useful: UW Madison I-131 brochure

    Bookmark   May 6, 2013 at 1:12PM
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My little cat was on Methimazole (1/2 of 5 mg 2x/day), but she was always throwing up, so I stopped giving it to her. She seems to be fine now, except that she is so thin. She'll go outside in the morning sometimes, eat grass and throw up then, but seems to be keeping what food she does eat down. Unfortunately, I had the iodine treatment scheduled, but had to cancel when I lost my job. Likewise, I can't afford to take her back to the vet. Should I reduce the Methimazole to 1/2 pill once a day - maybe 1/4 pill 2x/day?

    Bookmark   May 8, 2013 at 10:12AM
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Ok..... I will just say it straight out....Do not use methimazole.. My cat was on it for less than 3 months and died from the effects of it. Even before she died, the drug changed her in a bad way. She was lethargic, she threw up a lot. She was weak from loss of appetite. Her personality changed dramatically.

If i had to do it all over again. I would give her only a small dose once a day.The drug was too toxic for her. She was under care of a vet and was recommended to give her 5mg twice a day. I later found out most vets recommend a much lesser dosage than the 5mg's twice a day.

I get to live and cat does not.......

    Bookmark   October 7, 2013 at 10:42PM
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